Chemoattractant cytokines (chemokines) are important mediators of lymphocyte trafficking from the circulation into sites of tissue damage and inflammation, and into secondary lymphoid organs. Certain chemokines rapidly trigger lymphocyte integrins, causing increased avidity with endothelial ligands, resulting in arrest of the lymphocyte on endothelial cells close to the chemokine source. Gradients of chemokine molecules can also attract arrested cells through the endothelium and into the surrounding tissue. A variety of chemokines are differentially expressed in various tissue types, suggesting a role in the differential homing of specific lymphocyte subsets to various types of tissue. We have found (in the human system) that the chemokines TARC and MDC efficiently attract circulating systemic memory T cells, especially skin-homing T cells expressing the cutaneous lymphocyte antigen, CLA. In contrast, intestinal (a4B7+) memory and naive T cells respond poorly. Immunohistochemistry reveals anti-TARC reactivity with venules involved in lymphocyte trafficking in chronically inflamed skin, but not in the gastrointestinal lamina propria, suggesting a potential role in circulating CLA+ lymphocyte recognition of skin vasculature. Consistent with this, TARC triggers integrin-dependent adhesion of CLA+ (but not a4B7hi intestinal) memory T cells to ICAM-1; and mediates rapid integrin-dependent arrest of lymphocytes rolling on the vascular CLA receptor, E-selectin, under physiologic flow conditions. The results suggest a fundamental role for TARC and its lymphocyte receptor CCR4 in lymphocyte-endothelial cell recognition and in differential trafficking of lymphocyte populations responsible for systemic vs. intestinal immunity. In order to define this role in detail, here we shall characterize CCR4 expression and responsiveness to TARC and MDC among specialized subsets of lymphocytes in man (Aim 1). We will determine if the preferential effects of TARC and MDC on systemic vs. mucosal lymphocytes are also observed in the mouse, and will ask if these responses are CCR4-dependent (Aim 2). The availability of a mutant CCR4-deficient mouse line will allow us to explore the role of this receptor in targeted lymphocyte homing to inflamed skin (Aim 3) and its role in the inflammation process in models of DTH and autoimmune psoriasis (Aim 4). Monoclonal antibodies to mouse CCR4 and its ligands will facilitate these studies (Aim 5). These studies promise to define a critical component of lymphocyte recruitment to systemic sites inflammation, and may lead to novel therapeutic approaches in cutaneous (i.e. psoriasis) and other inflammatory diseases.